Honeycomb structure

ABSTRACT

A honeycomb structure includes: a honeycomb structure body including a plurality of cells; and a plugging portion to alternately plug open end parts of the plurality of cells on one side as an inflow side of the exhaust gas and open end parts on the other side as an outflow side of the exhaust gas. The partition wall is loaded, on the side of the outflow cells, with an oxidation catalyst made of a transition metal oxide to oxidize NO gas or an oxidation catalyst made of a transition metal oxide loaded at CeO2 to oxidize NO gas. The partition wall has porosity of 70% or less, the oxidation catalyst has a particle diameter of more than 1 μm and less than 50.0 μm, and the loading amount of the oxidation catalyst is 5.0 g/L or more and 50 g/L or less.

“The present application is an application based on JP-2016-068417 filedon Mar. 30, 2016 with Japan Patent Office, the entire contents of whichare incorporated herein by reference.”

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a honeycomb structure that can be usedas an exhaust-gas purifying filter to purify exhaust gas.

Description of the Related Art

A plurality of filters are disposed at an exhaust path of a dieselengine or the like to purify exhaust gas emitted from the engine.Examples of the filters include a diesel particulate filter (DPF) and anSCR (Selective Catalytic Reduction) catalyst (catalyst to reduce acomponent to be purified through a reduction reaction in a selectedmanner) converter. The SCR catalyst converter is disposed downstream ofthe DPF. The DPF is configured to trap a particulate matter (PM) mainlyincluding soot in exhaust gas and prevent emission of the particulatematter into the air. The SCR catalyst converter is configured to reduceNO_(x) in the exhaust gas using ammonia (NH₃). This ammonia is generatedthrough decomposition of urea that is injected from a urea injectordisposed upstream of the SCR catalyst converter.

The DPF is typically loaded with a catalyst including noble metal tooxidize carbon monoxide (CO) and hydrocarbon (HC) for removal. That is,in the DPF, soot accumulated at the inside is burned for removal(combustion regeneration treatment). At this time, the catalyst promotesthe combustion of soot. The catalyst oxidizes carbon monoxide (CO) andhydrocarbon (HC) for removal that are generated through decomposition ofthe soot when the soot is burned.

Patent Document 1 is known, for example, as the patent applicationrelating to a urea SCR system. When NO_(x) is decomposed into N₂ and H₂Oby the SCR catalyst converter, the ratio of NO and NO₂ flowing into theSCR catalyst converter is preferably 1:1 in terms of the reaction speed.In the DPF (Diesel Oxidation Catalyst (DOC)+CSF (Catalyzed Soot Filter))upstream of the SCR catalyst converter, however, NO₂ is consumed for thecombustion of PM, so that the ratio of NO is currently considerably morethan NO₂ when they are emitted from the DPF. This means that the NO_(x)purifying efficiency is not good.

A technique to bring the ratio of NO and NO₂ flowing into the SCRcatalyst converter close to 1:1 has been demanded.

In order to bring the ratio of NO and NO₂ close to 1:1, a catalyst madeof noble metal such as Pt may be disposed at a latter part of the CSF soas to oxidize a part of NO into NO₂. Such a catalyst of noble metaloften leads to an increase in the cost. Since noble metal has highoxidizing power, it is difficult to control the amount of the noblemetal used to convert NO into NO₂.

Patent Document 2, which is the patent application relating to anexhaust-gas purifying system as a whole, discloses the configurationincluding CoO, MnO₂, or ZrO as an oxidation catalyst at a former part ofthe SCR catalyst converter.

Patent Documents 3 and 4 disclose a catalyst loaded honeycomb includingan oxidation catalyst. Patent Document 5 discloses an exhaust-gaspurifying apparatus including a catalyst to purify nitrogen oxide.

[Patent Document 1] JP-A-2004-100699

[Patent Document 2] JP-A-5-195756

[Patent Document 3] JP-A-2014-57951

[Patent Document 4] JP-A-2008-302355

[Patent Document 5] JP-A-2006-346605

SUMMARY OF THE INVENTION

In Patent Document 2, since CoO is not stable unless the temperature isat 800° C. or more, there is a concern about the stability in exhaustgas at low temperatures. Since MnO₂ is decomposed into Mn₂O₃ at 550° C.or more, there is a concern about the stability in exhaust gas at hightemperatures.

Patent Document 3 aims to promote the combustion action of soot andreduce the amount of soot accumulated at the wall of cells with time.Therefore the catalyst is too fine to function well to promote thereaction of NO and NO₂ in the SCR catalyst converter. Such a finecatalyst has a problem in durability because sintering occurs quickly.

Patent Document 4 has a problem in the strength because the porosity isvery high to achieve both of lower pressure loss and higher trappingefficiency of PM.

Patent Document 5 is a technique to purify NO_(x) by reduction, and theloading amount of a catalyst is large.

In order to promote the reaction of NO and NO₂ in the SCR catalystconverter, a simple technique at low cost has been demanded to convert apart of the NO into NO₂. The present invention aims to provide ahoneycomb structure that can be used as an exhaust-gas purifying filterto purify exhaust gas. Especially the present invention provides ahoneycomb structure that can be used as a CSF disposed upstream of a SCRcatalyst converter, and is configured to convert a part of NO to NO₂appropriately.

To fulfill the above aim, the present invention provides the followinghoneycomb structure.

According to a first aspect of the present invention, a honeycombstructure is provided, including: a honeycomb structure body including aporous partition wall having a large number of pores, the honeycombstructure body including a plurality of cells defined by the partitionwall and serving as a through channel of fluid; and a plugging portionto alternately plug open end parts of the plurality of cells on one sideas an inflow side of the exhaust gas and open end parts on the otherside as an outflow side of the exhaust gas, wherein the plurality ofcells includes inflow cells that are open at the open end parts on theinflow side and outflow cells that are open at the open end parts on theoutflow side, the partition wall has porosity of 70% or less, thepartition wall is loaded, on the side of the outflow cells, with anoxidation catalyst made of a transition metal oxide to oxidize NO gas oran oxidation catalyst made of a transition metal oxide loaded at CeO₂ tooxidize NO gas, the oxidation catalyst has a particle diameter of morethan 1 μm and less than 50.0 μm, and the loading amount of the oxidationcatalyst is 5.0 g/L or more and 50 g/L or less.

According to a second aspect of the present invention, the honeycombstructure according to the first aspect is provided, wherein thehoneycomb structure has a NO₂ conversion rate at 250° C. that is morethan 3.0% and less than 35%.

According to a third aspect of the present invention, the honeycombstructure according to the first or second aspects is provided, whereinthe transition metal oxide includes any one of Fe₂O₃, MnO₂, and Co₃O₄.

According to a fourth aspect of the present invention, the honeycombstructure according to the first or second aspects is provided, whereinthe oxidation catalyst made of the transition metal oxide loaded at CeO₂includes any one of Fe₂O₃/CeO₂, MnO₂/CeO₂, and Co₃O₄/CeO₂.

According to a fifth aspect of the present invention, the honeycombstructure according to any one of the first to fourth aspects isprovided, wherein 60% or more of the transition metal oxide is loaded atthe partition wall from a ½ position in the thickness direction towardthe side of the outflow cells.

The honeycomb structure of the present invention includes a pluggingportion to alternately plug open end parts of the plurality of cells onone side as an inflow side of the exhaust gas and open end parts on theother side as an outflow side of the exhaust gas. The honeycombstructure includes inflow cells into which exhaust gas flows and outflowcells from which the exhaust gas passed through the partition wall flowsout. The partition wall is loaded, on the side of the outflow cells,with an oxidation catalyst made of a transition metal oxide to oxidizeNO gas or an oxidation catalyst made of a transition metal oxide loadedat CeO₂ to oxidize NO gas. The partition wall has porosity of 70% orless, the oxidation catalyst has a particle diameter of more than 1 μmand less than 50.0 μm, and the loading amount of the oxidation catalystis 5.0 g/L or more and 50 g/L or less. Thereby, a good balance betweenNO and NO₂ flowing into the SCR catalyst converter disposed downstreamcan be obtained, and the reactions of NO, NO₂, and NH₃ occurefficiently. That is, the purifying efficiency of exhaust gas canincrease.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a honeycombstructure.

FIG. 2 is a schematic view of a cross section of the honeycomb structurethat is parallel to the extending direction of cells.

FIG. 3A is an enlarged cross-sectional view schematically showing a partof a cross section of the honeycomb structure that is parallel to theextending direction of cells.

FIG. 3B is an enlarged cross-sectional view schematically showing a partof a cross section of the honeycomb structure that is parallel to theextending direction of cells.

FIG. 4 schematically shows transition metal oxide loaded at CeO₂.

FIG. 5 schematically shows an exhaust-gas purifying system.

FIG. 6 describes reactions in the exhaust-gas purifying system.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The following describes embodiments of the present invention, withreference to the drawings. The present invention is not limited to thefollowing embodiments, and is susceptible to various changes,modifications and improvements without deviating from the scope of theinvention.

(1) Honeycomb Structure:

FIGS. 1 and 2 show one embodiment of a honeycomb structure of thepresent invention. FIG. 1 is a perspective view schematically showing ahoneycomb structure 1. FIG. 2 is a schematic view of a cross section ofthe honeycomb structure 1 that is parallel to the extending direction ofcells 3. The honeycomb structure 1 includes a honeycomb structure body10 having a porous partition wall 4 with a large number of pores, inwhich a plurality of cells 3 serving as a through channel of exhaust gasare defined by the partition wall 4, and a plugging portion 8 toalternately plug open end parts of the plurality of cells 3 on one sideas an inflow side of the exhaust gas and open end parts on the otherside as an outflow side of the exhaust gas. The plurality of cells 3includes inflow cells 3 a that are open at the open end parts on theinflow side and outflow cells 3 b that are open at the open end parts onthe outflow side. The plugging portion 8 includes an inflow sideplugging portion 8 a disposed at the inflow end face 2 a of apredetermined cells 3 and an outflow side plugging portion 8 b disposedat the outflow end face 2 b of the residual cells 3.

FIGS. 3A and 3B are enlarged views of region A in FIG. 2. The partitionwall has porosity of 70% or less, and as shown in FIGS. 3A and 3B, thepartition wall 4 is loaded, on the side of the outflow cells 3 b, withan oxidation catalyst 5 made of a transition metal oxide 5 a to oxidizeNO gas or an oxidation catalyst 5 made of a transition metal oxide 5 aloaded at CeO₂ to oxidize NO gas. The oxidation catalyst 5 has aparticle diameter of more than 1 μm and less than 50.0 μm. The loadingamount of the oxidation catalyst 5 is 5.0 g/L or more and 50 g/L orless. The honeycomb structure 1 can be used as an exhaust-gas purifyingfilter. NO included in exhaust gas flowing into the honeycomb structure1 is oxidized to NO₂. The honeycomb structure 1 preferably has a NO₂conversion rate at 250° C. that is more than 3.0% and less than 35%. TheNO₂ conversion rate is the ratio of NO that is converted into NO₂. Whenthe NO₂ conversion rate is within this range, exhaust gas flowing into adownstream SCR catalyst converter 70 (see FIG. 5) can be purifiedefficiently.

The transition metal oxide 5 a loaded at the partition wall 4 ispreferably any one of Fe₂O₃, MnO₂, and Co₃O₄. These transition metaloxides 5 a enable appropriate oxidization of NO into NO₂. Therefore theloading amount of catalyst can be adjusted easily. Especially Fe₂O₃ andCo₃O₄ are stable substances at 200 to 800° C., and there is no concernabout the stability.

The oxidation catalyst 5 made of the transition metal oxide 5 a loadedat CeO₂ is preferably any one of Fe₂O₃/CeO₂, MnO₂/CeO₂, and Co₃O₄/CeO₂.As shown in FIG. 4, Fe₂O₃/CeO₂ is an oxidation catalyst 5 in which CeO₂is loaded with Fe₂O₃. Similarly, MnO₂/CeO₂ and Co₃O₄/CeO₂ also arecatalysts in which CeO₂ is loaded with MnO₂ and Co₃O₄, respectively.

The oxidation catalyst has a particle diameter of more than 1 μm andless than 50.0 μm, preferably more than 2 μm and less than 30.0 μm, andmore preferably more than 3 μm and less than 15.0 μm. In the presentdescription, a particle diameter is obtained as follows. Firstly, animage (image of a raw material) in a visual field with 1000magnification is observed with a SEM. Three of the visual fields areobserved. In each visual field, all particulates included in the visualfield are targets of the observation. Then, for all of the particulatesin the three visual fields, the average of the three visual fields as awhole is found, and this value is set as the particle diameter.

The loading amount of the oxidation catalyst is 5.0 g/L or more and 50g/L or less, preferably 5.0 g/L or more and 45 g/L or less, and morepreferably 5.0 g/L or more and 40 g/L or less. The loading amount thecatalyst (g/L) in the present description is the amount (g) of thecatalyst that is loaded per unit volume (L) of the honeycomb structure.

Preferably, 60% or more of the transition metal oxide 5 a is loaded atthe partition wall 4 from a ½ position in the thickness direction towardthe side of the outflow cells 3 b. FIG. 3A schematically shows that 60%or more of transition metal is loaded at the partition wall 4 from a ½position in the thickness direction toward the side of the outflow cells3 b. FIG. 3B schematically shows that transition metal is loaded on thesurface and the surrounding of the partition wall 4 on the side of theoutflow cell 3 b. In the present description, the loading amount of thetransition metal oxide 5 a is measured as follows. Firstly a porousmaterial of the present invention surrounded with resin ismirror-polished using diamond slurry or the like, which is a sample tobe observed. This cross-sectional polished face is observed with 100magnification to obtain a photo of a microstructure. In this photo, thenumber of all catalyst particulates is measured, and this is the numberof all catalyst particulates N_(a). Next, the number of the catalystparticulates from a half of the partition wall to the outlet ismeasured, and this is the number of catalyst N_(h) occupying ½ of thethickness of the partition wall. The ratio of the catalyst occupying ½of the thickness of the partition wall 4 is calculated by N_(h)/N_(a)from these measurements.

When exhaust gas passes through the partition wall 4 of such a honeycombstructure 1, a part of NO is oxidized by the transition metal oxide 5 aand is changed into NO₂. Thereby, a good balance between NO and NO₂ canbe obtained, which flow into a downstream SCR catalyst converter 70.This can promote a chemical reaction to purify the exhaust gas and sothe exhaust gas can be purified efficiently.

A part of the partition wall 4 close to the inflow cell 3 a in thethickness direction is preferably loaded with a catalyst to promote thecombustion of soot. As such a catalyst to promote the combustion ofsoot, a catalyst including noble metal at the ratio of about 5 to 30 g/Lpreferably is loaded. In the present specification, the description thata catalyst is loaded at a part close to the inflow cell 3 a refers tothat the amount of the catalyst including noble metal loaded from thesurface of the partition wall 4 on the side of the inflow cell to ½ ofthe thickness accounts for 60% or more of the entire amount.

Exhaust gas passing through the honeycomb structure 1 of the presentinvention is fed to the SCR catalyst converter 70 disposed downstream ofthe honeycomb structure 1. In this SCR catalyst converter 70, NO_(x) inthe exhaust gas can be favorably purified using NO, NO₂, and ammonia.That is, the SCR catalyst converter 70 is configured to purify NO_(x) inthe exhaust gas with urea-derived ammonia supplied by a urea injector60.

(1-1) Honeycomb Structure Body:

The partition wall 4 preferably has a thickness of 50 to 500 μm, morepreferably 100 to 450 μm, and particularly preferably 150 to 450 μm. Ifthe thickness of the partition wall 4 is the lower limit or more, thestrength of the honeycomb structure body is enough. If the thickness isthe upper limit or less, the pressure loss can be suppressed.

The partition wall 4 preferably has porosity of 25 to 70%, morepreferably 30 to 70%, and particularly preferably 34 to 68%. If theporosity is the lower limit or more, an increase in pressure loss can besuppressed. If the porosity is the upper limit or less, the honeycombstructure body can have enough strength. In order to obtain porosity ofthe partition wall 4, a SEM image of a cross section of the partitionwall 4 is taken with 1000 magnification or more. Binarization is thenperformed for the taken SEM image based on a difference in brightnessbetween a solid part and a void part. Next, the ratio of area betweenthe void part and the solid part is obtained at visual fields of 20 ormore, and the average of the ratio of area is calculated. This is theporosity of the partition wall.

The average pore diameter of the partition wall 4 preferably is 5 to 40μm, more preferably 8 to 30 μm, and particularly preferably 9 to 25 μm.If the average pore diameter of the partition wall 4 is the lower limitor more, an increase in pressure loss can be suppressed. If the averagepore diameter is the upper limit or less, the trapping efficiency ofsoot increases. The average pore diameter is calculated as follows.Firstly a SEM image of a cross section of the partition wall 4 is takenwith 1000 magnification or more. Binarization is then performed for thetaken SEM image based on a difference in brightness between a solid partand a void part. Next, a circle inscribed in the outline of a solid partin a void part is drawn at 20 positions or more, and the average of thediameters of these inscribed circles is calculated. This is the averagepore diameter.

The honeycomb structure body 10 preferably has a cell density of 8 to 95cells/cm², and 15 to 78 cells/cm² more preferably. If the cell densityis the lower limit or more, the filtering area is enough and so thetrapping efficiency of soot can increase. If the cell density is theupper limit or less, the pressure loss in the honeycomb structure bodywithout soot accumulated (initial pressure loss) can be suppressed.

A preferable example of the material of the honeycomb structure body 10includes ceramic. From the viewpoints of the strength, heat resistanceand corrosion resistance, for example, the honeycomb structure body ispreferably made of any one of cordierite, silicon carbide, alumina,mullite, aluminum titanate, silicon nitride and a silicon-siliconcarbide based composite material including silicon carbide as aggregateand metal silicon as a raw material of a binding part. Among them,cordierite is particularly preferable.

(1-2) Plugging Portion:

The honeycomb structure 1 includes an inflow side plugging portion 8 aand an outflow side plugging portion 8 b. The honeycomb structure havingthese plugging portions 8 enables the favorable trapping of particulatematter in exhaust gas. In the honeycomb structure body 10 of thehoneycomb structure 1, cells 3 having the outflow side plugging portion8 b are inflow cells 3 a, and the cells 3 having the inflow sideplugging portion 8 a are outflow cells 3 b.

The honeycomb structure 1 may have a length in the extending directionof the cells 3 that is 30 to 500 mm.

The honeycomb structure of the present invention may further include acircumferential wall 7 (see FIG. 1) on the lateral face of the honeycombstructure body 10.

The honeycomb structure body 10 may be a bonded member including aplurality of honeycomb segments. That is, the honeycomb structure body10 may include a collective body of these plurality of honeycombsegments and a bonding part made of a bonding material to bond thesehoneycomb segments.

The honeycomb structure 1 may have a configuration in which the inflowcells 3 a and the outflow cells 3 b have different shapes in a crosssection perpendicular to the extending direction of the cells 3 (HACconfiguration: High Ash Capacity configuration). For instance, they maybe the combination of octagons and quadrangles in a cross section,including inflow cells 3 a having a larger cell cross-sectional area andoutflow cells 3 b having a smaller cell cross-sectional area. This canincrease the surface area of the surface of the inflow cells 3 a inwhich particulate matter or the like accumulates, and so an increase inpressure loss can be suppressed.

(2) Method for Manufacturing Honeycomb Structure:

The following describes a method for manufacturing the honeycombstructure 1 of the present embodiment. Firstly a kneaded material isprepared to produce the honeycomb structure 1. This kneaded material isformed to have a honeycomb formed body (forming step). Thereafterplugging is performed at open ends of predetermined cells 3 at theinflow end face and at open ends of the residual cells 3 at the outflowend face to form an inflow side plugging portion and an outflow sideplugging portion (plugging step). Thereafter, the honeycomb formed bodyhaving the plugging portions 8 formed alternately is fired to have ahoneycomb fired body (firing step). In this way, the honeycomb structure1 can be manufactured.

A catalyst may be loaded before forming the plugging portion 8 or may beloaded after forming the plugging portion 8. The following describeseach manufacturing step in details.

(2-1) Forming Step:

In the forming step, a kneaded material including a ceramic forming rawmaterial containing a ceramic raw material is prepared. This kneadedmaterial is formed to have a honeycomb formed body in which a pluralityof cells 3 are defined. The plurality of cells serves as a throughchannel of fluid.

This ceramic forming raw material is preferably prepared by mixing theceramic raw material as stated above with dispersing medium, organicbinder, inorganic binder, pore former, surfactant and the like. Thecomposition ratio of these raw materials is not limited especially, anda composition ratio suitable for the structure of the honeycombstructure 1 to be manufactured, its materials and the like ispreferable.

A method for preparing a kneaded material is not limited especially. Forinstance, a kneader or a vacuum pugmill may be used for this purpose. Asa method for forming the kneaded material, a conventionally knownforming method can be used, such as extrusion and injection molding. Apreferable example of the method for this includes a method for forminga honeycomb formed body by extrusion using a die having a desired cellshape, partition wall thickness and cell density.

Examples of the shape of the honeycomb formed body include a pillarshape having a cross section orthogonal to the center axis that is of acircle shape, an ellipse shape, a race-track shape, a triangle shape, aquadrangle shape, a pentagon shape, a hexagon shape and an octagonshape.

The obtained honeycomb formed body may be dried. Examples of the methodfor drying include hot air drying, microwave drying, dielectric drying,reduced-pressure drying, vacuum drying, and freeze-drying. Among them,dielectric drying, microwave drying or hot air drying is preferablyperformed alone or in combination.

(2-2) Firing Step:

Before firing (main firing) the honeycomb formed body, it is preferablethat the honeycomb formed body be calcinated. Calcination is fordegreasing. The method therefor is not limited especially as long as itcan remove internal organic substance (organic binder, dispersing agent,pore former and the like). In general the combustion temperature oforganic binder is about 100 to 300° C., and the combustion temperatureof pore former is 200 to 800° C. Therefore, the calcination ispreferably performed under the conditions at about 200 to 1000° C. forabout 3 to 100 hours in the air.

Suitable conditions may be selected for firing (main firing) of thehoneycomb formed body. For instance, a preferable firing temperature isfrom 1410 to 1440° C. The firing time is preferably 4 to 7 hours, whichis a time to keep the highest temperature.

(2-3) Plugging Step:

The plugging portion 8 may be formed by disposing a mask at open ends onone side of predetermined cells 3 and filling open ends of the residualcells 3 with plugging slurry. Such a method for forming the pluggingportion 8 can follow a conventionally-known method for manufacturing aplugging portion 8 of a honeycomb structure.

A raw material of the plugging portion 8 may be a raw material similarto that of the honeycomb structure body 10. This allows the expansionrate during the firing of the honeycomb formed body and the pluggingportion 8 to be the same. Therefore durability of the honeycombstructure 1 can increase.

(2-4) Loading with Catalyst

The oxidation catalyst 5 may be loaded on the side of the outflow cells3 b of the partition wall 4 of the honeycomb structure 1 by immersingthe honeycomb structure 1 having the plugging portion 8 into thecontainer storing slurry of the oxidation catalyst 5 from the outflowend face 2 b. Then, the slurry is sucked from the side of the inflowcells 3 a. When the honeycomb structure still does not have the pluggingportion 8, similar procedure may be performed after attaching a maskhaving holes at positions corresponding to the predetermined cells 3 tothe end face of the honeycomb structure 1.

The viscosity of the slurry of the oxidation catalyst 5, the particlediameter of the oxidation catalyst 5 included and the suction power tosuck the slurry may be adjusted, so that catalyst can be loaded not onlyat the surface of the partition wall 4 but also at the inside of poresof the partition wall 4. The amount of the catalyst loaded also can beadjusted. Suction of the slurry may be performed a plurality of times,whereby the amount of the catalyst loaded can be adjusted.

(3) Exhaust-Gas Purifying System:

FIG. 5 shows an exhaust-gas purifying system 100 including a DPF 30 anda SCR catalyst converter 70. The DPF 30 further includes a DOC 40(upstream-side oxidation catalyst) and a CSF 50. The honeycomb structure1 of the present invention can be used as the CSF 50. The exhaust-gaspurifying system 100 includes a urea injector 60 between the DPF 30 andthe SCR catalyst converter 70.

The SCR catalyst converter 70 is disposed downstream of the CSF 50(honeycomb structure 1), and is a filter loaded with SCR catalyst. TheDOC 40 (upstream-side oxidation catalyst) is disposed upstream of theCSF 50 (honeycomb structure 1), and is a filter loaded with oxidationcatalyst. The urea injector 60 enables injection of urea, and isdisposed between the honeycomb structure 1 and the SCR catalystconverter 70.

The exhaust-gas purifying system 100 is to purify exhaust gas emittedfrom an engine. Exhaust gas emitted from the engine passes through theDPF 30 (DOC 40, CSF 50), and then flows into the SCR catalyst converter70 together with urea for purification. The exhaust-gas purifying system100 may include a downstream-side oxidation catalyst 80 downstream ofthe SCR catalyst converter 70, and the downstream-side oxidationcatalyst 80 is to oxidize ammonia. Reactions in them are described asfollows, referring to FIG. 6.

When NO, O₂, and N₂ (I in FIG. 6) flow into the DOC 40, the followingreactions occur at the DOC 40, and NO₂ is generated (II in FIG. 6).2NO+O₂=2NO₂  (Formula 1)SOF+O₂═CO,CO₂,H₂O  (Formula 2)

SOF (Soluble Organic Fraction) is included in PM (particulate matter).

The DOC 40 (upstream-side oxidation catalyst) oxidizes NO and purifiesSOF. As the DOC 40, a well-known catalyst may be used as needed.Specifically the upstream-side oxidation catalyst includes apillar-shaped honeycomb structure having a partition wall that defines aplurality of cells serving as a through channel of fluid, and oxidationcatalyst loaded at the surface of the partition wall of this honeycombstructure.

In the CSF 50 (honeycomb structure 1), the following reactions occur, sothat NO is generated from NO₂ (III in FIG. 6)C (soot)+2NO₂═CO₂+2NO  (Formula 3)C (soot)+NO₂=CO+NO  (Formula 4)C (soot)+½O₂+NO₂=CO₂+NO  (Formula 5)

The urea injector 60 is to inject urea upstream of the SCR catalystconverter 70 so as to supply ammonia decomposed and generated from theurea to the SCR catalyst converter 70. As the urea injector 60, aconventionally known urea injector can be used, which can inject apredetermined amount of urea.

Gas emitted from the CSF 50 and urea injected from the urea injector 60flow into the SCR catalyst converter 70. Then the following reactionsoccur in the SCR catalyst converter 70, and the exhaust gas is purified(IV in FIG. 6).4NO+4NH₃+O₂=4N₂+6H₂O  (Formula 6)NO+NO₂+2NH₃=2N₂+3H₂O  (Formula 7)6NO₂+8NH₃=7N₂+12H₂O  (Formula 8)

The SCR catalyst converter 70 is to purify NO_(x) with ammonia that isgenerated through decomposition of urea injected from the urea injector60. As the SCR catalyst converter 70, a well-known one may be used.Specifically the SCR catalyst converter 70 includes a pillar-shapedhoneycomb structure having a partition wall that defines a plurality ofcells serving as a through channel of fluid, and SCR catalyst loaded atthe surface of the partition wall of this honeycomb structure.

As shown in Formula 7, NO and NO₂ react at 1:1, so that N₂ and H₂O aregenerated. NO and NO₂ flowing into the SCR catalyst converter 70 are1:1, which is necessary for efficient reactions in the SCR catalystconverter 70.

The honeycomb structure used for the CSF 50 of the present inventionincludes the oxidation catalyst 5 made of transition metal oxide 5 a tooxidize NO gas or the oxidation catalyst 5 made of transition metaloxide 5 a loaded at CeO₂ to oxidize NO gas, and these catalyst areloaded on the side of the outflow cells 3 b of the partition wall 4.This can convert a part of NO emitted from the CSF 50 into NO₂ (III inFIG. 6, NO→NO₂). Thereby, a good balance between NO and NO₂ flowing intothe SCR catalyst converter 70 can be obtained, and the reactions of NO,NO₂, and NH₃ occur efficiently.

The exhaust-gas purifying system 100 of FIG. 5 further includes thedownstream-side oxidation catalyst 80. The downstream-side oxidationcatalyst 80 is disposed downstream of the SCR catalyst converter 70, andis a honeycomb structure loaded with oxidation catalyst. There is a fearthat ammonia may be emitted from the SCR catalyst converter 70 to theair. Such a downstream-side oxidation catalyst 80, however, can oxidizeand remove ammonia emitted from the SCR catalyst converter 70.

The downstream-side oxidation catalyst 80 used may be the same as theupstream-side oxidation catalyst as stated above. Specifically thedownstream-side oxidation catalyst 80 includes a pillar-shaped honeycombstructure having a partition wall that defines a plurality of cellsserving as a through channel of fluid, and oxidation catalyst loaded atthe surface of the partition wall of this honeycomb structure.

With this configuration, the exhaust-gas purifying system 100 includingthe honeycomb structure 1 of the present invention enables efficientreactions of NO, NO₂, and NH₃ and so the purifying efficiency of exhaustgas increases.

EXAMPLES

The following describes the present invention by way of examples in moredetails. The present invention is not limited to these examples.

Example 1

Pore former, organic binder and water were added to a cordierite formingraw material including talc, kaolin, and alumina to prepare a formingraw material. As the pore former, hollow resin particulates having theaverage particle diameter of 20 μm were used. This average particlediameter was a value measured by a laser diffraction method.Methylcellulose and hydroxypropoxyl methylcellulose were used as theorganic binder. The amount of these raw materials added was 15 parts bymass of the pore former, 4 parts by mass of the organic binder and 27parts by mass of water with reference to 100 parts by mass of thecordierite forming raw material.

Next, the forming raw material was kneaded to have a round pillar-shapedkneaded material. Next, the obtained round-pillar shaped kneadedmaterial was formed using a vacuum extruder to be a honeycomb shape, anda honeycomb formed body was formed in this way. The obtained honeycombformed body was dried by a microwave dryer, and then was dried by ahot-air dryer, and a honeycomb dried body was obtained in this way.

Next, a plugging portion 8 was formed at open end parts of the cells 3of this honeycomb dried body on one side. The plugging portion 8 wasformed so that a checkerboard pattern appeared at each of the end facesof the honeycomb dried body (inflow end face and outflow end face) withthe cells 3 having the plugging portion 8 at the open end parts and thecells 3 not having the plugging portion 8 at the open end parts. Theplugging portion 8 was formed firstly by attaching a sheet to the endface of the honeycomb dried body and boring holes at positions of thesheet corresponding to the cells 3 to which the plugging portion 8 wasto be formed. Next, while leaving this sheet attached there, the endface of the honeycomb dried body was immersed in slurry for plugging tofill the open ends of the cells 3 to which the plugging portion was tobe formed with the slurry for plugging via the holes of the sheet. Theslurry for plugging was a material of the plugging portion 8 in a slurryform. The material of the plugging portion 8 used was the same as theforming raw material as stated above.

The slurry for plugging filled in the open end parts of the cells 3 inthis way was dried. After that, this honeycomb dried body was calcinated(degreased) at 550° C. for 3 hours in the air. After that, this wasfired at about 1400 to 1500° C. for 7 hours, so that a honeycombstructure was obtained. This honeycomb structure had a cylindrical shapeof 144 mm in diameter and 152 mm in length. The cell shape was square,the cell density was 47 cells/cm², the thickness (T) of the partitionwall as a whole was 300 μm, the porosity of the partition wall as awhole was 41.0%, and the average pore diameter of the partition wall asa whole was 20 μm.

Subsequently, oxidation catalyst 5 made of transition metal oxide 5 a(Fe₂O₃ loaded at the carrier of CeO₂) was loaded at the surface of thepartition wall of this honeycomb structure on the outflow side.Specifically for this loading, firstly catalyst slurry was prepared,including the oxidation catalyst 5 made of the transition metal oxide 5a. Water was used as the dispersing agent of the catalyst slurry. Theamount of water was adjusted so that the slurry had viscosity of 7mPa·s. This catalyst slurry was introduced into the outflow cells 3 b ofthe honeycomb structure 1, and was sucked from the side of the inflowend face of the honeycomb structure 1 so as to coat the partition wall 4on the side of the outflow cells 3 b with the catalyst slurry.Subsequently, this honeycomb structure 1 was dried by a hot air dryer,so that a honeycomb structure 1 of Example 1 including the oxidationcatalyst 5 made of the transition metal oxide 5 a loaded was obtained.

Examples 2 to 10, Comparative Examples 1 to 3

Similarly to Example 1, honeycomb structures 1 loaded with the oxidationcatalyst 5 made of transition metal oxide 5 a of Examples 2 to 10 andComparative Examples 1 to 3 were obtained. Table 1 shows the details. InComparative Example 3, Pt was used instead of the oxidation catalyst 5.

(1) Open Porosity and Average Pore Diameter of Base Material

In the present description, open porosity was a value obtained bycalculation using the total pore volume (unit: cm³/g) in accordance withmercury intrusion porosimetry (complying with JIS R 1655) and apparentdensity (unit: g/cm³) measured by the Archimedes's method. The openporosity was calculated using the expression of “porosity [%]=total porevolume/{(1/apparent density)+total pore volume}×100”. The average porediameter was a value measured by mercury intrusion porosimetry(complying with JIS R 1655).

(2) Crystalline Phase of Catalyst

Crystalline phase was identified as follows. An X-ray diffractionpattern was obtained using an X-ray diffractometer. A rotatinganticathode X-ray diffractometer (RINT, manufactured by RigakuCorporation) was used as the X-ray diffractometer. X-ray diffraction wasmeasured using CuKα-ray source and at 50 kV, 300 mA and 2θ=10 to 60°.The X-ray diffraction data was analyzed using “X-ray data analysissoftware JADE7” manufactured by MDI to identify the crystalline phase.

(3) Particle Diameter of Catalyst

Particle diameter of the catalyst was measured as follows. A porousmaterial of the present invention surrounded with resin wasmirror-polished using diamond slurry or the like, which was a sample tobe observed. This cross-sectional polished face was observed with 1000magnification to obtain a photo of a microstructure. In this photo, allcatalyst particulates were measured. This measurement was performed for3 visual fields, and their average was calculated. This average was theparticle diameter of the catalyst.

(4) NO₂ Conversion Rate

The honeycomb structures manufactured as stated above (Examples 1 to 10and Comparative Examples 1 to 3) were processed, and test pieces of 25.4mm in diameter and 50.8 mm in length were obtained. The processedcircumference was coated with the same material as the base material.They were used as measurement samples for the evaluation using ananalyzer of exhaust gas from automobile. At this time, the measurementsample was set in a reaction tube in a heating furnace so that thetemperature was held at 250° C. Meanwhile mixture gas was prepared sothat the gas temperature was adjusted at 250° C., having a nitrogenbalance of NO (nitrogen monoxide): 200 ppm and O₂ (oxygen): 10%. Thismixture gas was introduced into the measurement sample set in thereaction tube. Gas emitted from the measurement sample (exhaust gas) wasanalyzed using an exhaust-gas measurement apparatus (manufactured byHORIBA, Ltd., MEXA-6000FT), to measure the exhaust concentration (NOconcentration and NO₂ concentration). Based on the measurement result,NO₂ conversion rate (1−(NO concentration/(NO concentration+NO₂concentration))) was calculated.

(5) Ratio of Catalyst Occupying ½ of the Thickness of the Partition Wall

Ratio of catalyst occupying ½ of the thickness of the partition wall wasmeasured as follows. A porous material of the present inventionsurrounded with resin was mirror-polished using diamond slurry or thelike, which was a sample to be observed. This cross-sectional polishedface was observed with 100 magnification to obtain a photo of amicrostructure. In this photo, the number of all catalyst particulateswas measured, and this was the number of all catalyst particulatesN_(a). Next, the number of the catalyst particulates from a half of thepartition wall to the outlet side was measured, and this was the numberof catalyst N_(h) occupying ½ of the thickness of the partition wall.The ratio of the catalyst occupying ½ of the thickness of the partitionwall was calculated by N_(h)/N_(a) from these measurements.

TABLE 1 Ratio of Loading catalyst Average amount occupying Porosity poreCrystalline of Specific Loading Crystallite NO₂ ½ of of diameter phaseof Catalyst catalyst Catalyst surface amount diameter conversionpartition Type of base of base catalyst synthesis at base particle areaof of active of active rate wall base material material activetemperature material diameter catalyst species species (250° C.)thickness material % μm species carrier ° C. g/L μm m²/g mass % nm % %Ex. 1 Si 41.0 11.0 Fe₂O₃ CeO₂ 300 20 4.8 112.5 5 13.5 3.5 Ex. 2 bonded300 20 4.5 100.0 20 13.0 4.0 Ex. 3 SiC 300 20 5.2 97.3 40 20.0 4.3 Ex. 4Co₃O₄ CeO₂ 300 10 5.0 84.2 20 13.9 6.3 Ex. 5 300 20 5.0 84.2 20 13.915.8 Ex. 6 500 20 5.1 78.0 20 19.9 13.4 Ex. 7 300 10 5.0 84.2 20 13.97.5 69.8 Ex. 8 20 14.1 67.0 Ex. 9 MnO₂ CeO₂ 300 20 4.6 100.0 20 30.0 8.0Ex. 10 Mn₂O₃ CeO₂ 550 20 5.0 98.0 20 35.0 12.0 Comp. Si 41.0 11.0 Co₃O₄SiO₂ 300 20 50.0 451.5 20 15.0 under Ex. 1 bonded measurement SiC limitComp. Si 41.0 11.0 Co₃O₄ Al₂O₃ 300 20 <0.05 142.3 20 18.0 under Ex. 2bonded measurement SiC limit Comp. Si 41.0 11.0 Pt Al₂O₃ 550 20 <0.05159.6 10 10.0 40.8 Ex. 3 bonded SiC

In Comparative Example 1 having the particle diameter of the catalyst of50.0 μm and Comparative Example 2 having the particle diameter less than0.05 μm, their NO₂ conversion rates were under the measurement limit.Since Comparative Example 3 included Pt as the catalyst, the NO₂conversion rate was too large.

In Examples 1 to 10, their NO₂ conversion rates were within theappropriate range (more than 3.0% and less than 35%).

The honeycomb structure of the present invention can be used as anexhaust-gas purifying filter to purify exhaust gas emitted from aninternal combustion engine such as a diesel engine or various types ofcombustion apparatuses. Especially the honeycomb structure of thepresent invention can be used as a particulate trapping filter to trapPM in the exhaust gas. Specifically the honeycomb structure of thepresent invention can be used as a CSF disposed upstream of a SCRcatalyst converter.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1: honeycomb structure, 2: end face, 2 a: inflow side end face,        2 b: outflow end face, 3: cell, 3 a: inflow cell, 3 b: outflow        cell, 4: partition wall, 5: oxidation catalyst, 5 a: transition        metal oxide, 7: circumferential wall, 8: plugging portion, 8 a:        inflow side plugging portion, 8 b: outflow side plugging        portion, 10: honeycomb structure body, 30: DPF, 40: DOC        (upstream-side oxidation catalyst), 50: CSF, 60: urea injector,        70: SCR catalyst converter, 80 downstream-side oxidation        catalyst, 100: exhaust-gas purifying system

What is claimed is:
 1. A honeycomb structure, comprising: a honeycombstructure body including a porous partition wall having pores, thehoneycomb structure body including a plurality of cells defined by thepartition wall and serving as a through channel of an exhaust gas; and aplugging portion to alternately plug open end parts of the plurality ofcells on one side as an inflow side of the exhaust gas and open endparts on the other side as an outflow side of the exhaust gas, whereinthe plurality of cells includes inflow cells that are open at the openend parts on the inflow side and outflow cells that are open at the openend parts on the outflow side, the partition wall has porosity of 70% orless, the partition wall is loaded, on the side of the outflow cells,with an oxidation catalyst consisting of a transition metal oxide loadedon CeO₂ to oxidize NO gas, wherein the transition metal oxide isselected from the group consisting of MnO₂ and Co₃O₄, the oxidationcatalyst has a particle diameter of more than 1 μm and less than 50.0μm, and the loading amount of the oxidation catalyst is 5.0 g/L or moreand 50 g/L or less, wherein 60% or more of the transition metal oxide isloaded only on the partition wall from a ½ position in the thicknessdirection toward the side of the outflow cells.
 2. The honeycombstructure according to claim 1, wherein the honeycomb structure has aNO₂ conversion rate at 250° C. that is more than 3.0% and less than 35%.3. The honeycomb structure according to claim 1, wherein the oxidationcatalyst has a specific surface area of from 78 m²/g to 112.5 m²/g. 4.The honeycomb structure according to claim 1, wherein 67% or more of thetransition metal oxide is loaded only on the partition wall from a ½position in the thickness direction toward the side of the outflowcells.
 5. The honeycomb structure according to claim 1, wherein 69.8% ormore of the transition metal oxide is loaded only on the partition wallfrom a ½ position in the thickness direction toward the side of theoutflow cells.